Among display devices, electroluminescence devices (EL devices) are self-luminescent display devices showing the advantage of wide angle of view, excellent contrast and rapid response rate. Eastman Kodak developed in 1987 an organic EL device which employs a low molecular weight aromatic diamine and an aluminum complex as material for forming an EL layer, for the first time [Appl. Phys. Lett. 51, 913, 1987].
The most important factor to determine luminous efficiency, lifetime or the like in an organic EL device is electroluminescent material. Several properties required for such electroluminescent materials include that the material should have high fluorescent quantum yield in solid state and high mobility of electrons and holes, is not easily decomposed during vapor-deposition in vacuo, and forms uniform and stable thin film.
Organic electroluminescent materials can be generally classified into high-molecular materials and low-molecular materials. The low-molecular materials include metal complexes and thoroughly organic electroluminescent materials which do not contain metal, from the aspect of molecular structure. Such electroluminescent materials include chelate complexes such as tris(8-quinolinolato)aluminum complexes, coumarin derivatives, tetraphenylbutadiene derivatives, bis(styrylarylene) derivatives, oxadiazole derivatives. From those materials, it is reported that light emission of visible region from blue to red can be obtained; and realization of full-colored display devices is expected thereby.
In order to realize a full-colored OLED display, three EL materials (red, green and blue) are employed, and development of those EL materials having high efficiency and long life is a significant subject to enhance the features of the overall organic electroluminescence. EL materials can be functionally classified into host materials and dopant materials. It is generally known that a device structure having the most excellent EL properties can be fabricated with an EL layer prepared by doping a dopant to a host. Recently, development of organic EL devices with high efficiency and long life comes to the fore as an urgent subject, and particularly urgent is development of a material with far better EL properties as compared to conventional EL materials as considering EL properties required for medium to large sized OLED panels.
In the meanwhile, for conventional blue materials, a number of materials have been developed and commercialized since the development of diphenylvinyl-biphenyl (DPVBi) (Compound a) by Idemitsu-Kosan. In addition to the blue material system from Idemitsu-Kosan, dinaphthylanthracene (DNA) (Compound b), tetra(t-butyl)perylene (Compound c) system or the like have been known. However, extensive research and development should be performed with respect to these materials. The distryl compound system of Idemitsu-Kosan, which is known to have highest efficiency up to now, has 6 μm/W of power efficiency and beneficial device lifetime of more than 30,000 hr. However, when it is applied to a full-colored display, the lifetime is merely several thousand hours, owing to decrease of color purity over operation time. In case of blue electroluminescence, it becomes advantageous from the aspect of the luminous efficiency, if the electroluminescent wavelength is shifted a little toward longer wavelength. However, it is not easy to apply the material to a display of high quality because of unsatisfactory color purity in blue. Furthermore, the research and development of such materials are urgent because of the problems in color purity, efficiency and thermal stability.

In the meanwhile, according to a patent application of Mitsui Chemicals (Japan) (U.S. Pat. No. 7,166,240), the compounds shown below have the absorption spectra at 390 to 430 nm, with luminous efficiency of 4.6 cd/A. However, embodiment of pure blue color is impossible with the symmetrical structure of the Patent Publication, and the material, which cannot provide pure blue luminescence, is inadequate to be practically applied to a full-colored display.

Representatives for conventional electron transport material include aluminum complexes such as tris(8-hydroxyquinoline)aluminum (III) (Alq), which has been used prior to the multilayer thin film OLED's disclosed by Kodak in 1987; and beryllium complexes such as bis(10-hydroxybenzo-[h]quinolinato)beryllium (Bebq), which was reported in the middle of 1990's in Japan [T. Sato et al., J. Mater. Chem. 10 (2000) 1151]. However, the limitation of the materials has come to the fore as OLED's have been practically used since 2002. Thereafter, many electron transport materials of high performance have been investigated and reported to approach their practical use.

In the meanwhile, non-metal complex electron transport materials of good features which have been reported up to the present include spiro-PBD [N. Johansson et al., Adv. Mater. 10 (1998) 1136], PyPySPyPy [M. Uchida et al., Chem. Mater. 13 (2001) 2680] and TPBI [Y.-T. Tao et al., Appl. Phys. Lett. 77 (2000) 1575] of Kodak. However, there remain various needs for improvement in terms of electroluminescent properties and lifetime.

Particularly noticeable is that conventional electron transport materials have only slightly improved operation voltage as compared to what was reported, or show the problem of considerable reduction of device operation lifetime. In addition, the materials exhibit adverse effects such as deviation in device lifetime for each color and deterioration of thermal stability. Up to the present, those adverse effects are in the way to achieve the objects such as reasonable power consumption and increased luminance, which have been the issues in manufacturing large-sized OLED panels.
As a host material for phosphorescent light emitting material, 4,4′-N,N′-dicarbazole-biphenyl (CBP) has been most widely known up to the present, and OLED's having high efficiency to which a hole blocking layer (such as BCP and BAlq) had been applied have been developed. Pioneer (Japan) or the like reported OLED's of high performances which were developed by using bis(2-methyl-8-quinolinato)(p-phenylphenolato)aluminum (III) (BAlq) derivatives as the host.

Though the conventional materials are advantageous in view of light emitting property, they have low glass transition temperature and very poor thermal stability, so that the materials tend to be changed during high temperature vapor-deposition in vacuo. In an organic electroluminescent device (OLED), it is defined that power efficiency=(π/voltage)×current efficiency. Thus, the power efficiency is inversely proportional to the voltage, and the power efficiency should be higher in order to obtain lower power consumption of an OLED. In practice, an OLED employing phosphorescent electroluminescent (EL) material shows significantly higher current efficiency (cd/A) than an OLED employing fluorescent EL material. However, in case that a conventional material such as BAlq and CBP as host material of the phosphorescent EL material is employed, no significant advantage can be obtained in terms of power efficiency (lm/w) because of higher operating voltage as compared to an OLED employing a fluorescent material.
Furthermore, there was no satisfactory result in view of life of an OLED, so that development of host material providing better stability and higher performance is still required.